Chromized steel product

ABSTRACT

Chromized steel product having carbon content as low as possible, carbon stabilizer in atomic excess of carbon and 2.06.0 wt. % nickel or 0.6-6.0 wt. % manganese to improve strength, decrease grain size and give transformation-induced dislocation substructure upon cooling. Nickel improves toughness. Up to 1.0 wt. % titanium, as carbon stabilizer, improves strength. Chromizing process includes heating steel base in sealed retort with ferrochrome alloy and halide at 1,800*-1,850 F. for 6-8 hours and providing composition and cooling rate giving a transformation-induced dislocation substructure, while avoiding oxidation of chromium surface.

United States Patent 1 Heitmann 1 Feb. 20, 1973 abandoned.

[52] US. Cl. ..29/196.6, 75/122 K [51] Int. Cl. ..B32b 15/00 [58] Fieldof Search ..29/196.6; 75/123 K, 123 J 1,480,706 Yensen ..75/123 12,051,991 8/1936 Fleming... ....75/123 J 2,736,648 1/1956 Eckel ..75/123M 2,962,391 1/1960 Samuel ....29/196.6 3,312,546 4/1967 Mayer ..29/l96.6

Primary Examiner-Hyland Bizot Attorney-Merriam, Marshall, Shapiro &Klose 5 7 ABSTRACT Chromized steel product having carbon content as lowas possible, carbon stabilizer in atomic excess of carbon and 2.0-6.0wt. nickel or 0.6-6.0 wt. manganese to improve strength, decrease grainsize and give transformation-induced dislocation substructure uponcooling. Nickel improves toughness. Up to 1.0 improves [56] ReferencesCited wt. titanium, as carbon stabilizer,

strength. UNITED STATES PATENTS Chromizing process includes heatingsteel base in 2,643,949 6/1953 Jisdale ..75/123K al d retort withferrochrome alloy and halide at 2,670,281 2/1954 Hutchison.. ..75/123 K1,8001,850 F. for 68 hours and providing com- 3,184,33l 5/1965 Carter..29/196.6 position and cooling rate giving a transformation-in- J J1965 Durham 196-6 duced dislocation substructure, while avoiding oxida-3,249,456 5/1966 Carosel1a.... ..29/196.6 {ion of chromium Surface3,353,936 11/1967 Samuel ..29/196.6 3,355,265 11 1967 Hudson ..29/196.610 Claims, 3 Drawing Figures I P (D 5 0 AIR coouao FROM |so0 F O I I dFURNACE COOLED FROM |a0o F WEIGHT PCT TITANIUM CHROMIZED STEEL PRODUCTCROSS REFERENCE TO RELATED APPLICATION This is a continuation ofapplication Ser. No. 653,161 filedJuly 13, 1967.

BACKGROUND OF THE INVENTION The present invention relates generally tochromized steel products, and more particularly to a chromized steelproduct of increased strength and toughness and to a process forproducing the improved product.

As discussed herein, a chromized steel product has a steel base and adiffusion alloyed chromium surface resulting from a process in whichiron atoms on the surface of the product have been replaced by chromiumatoms. The chromium surface improves the corrosion resistance of theproduct.

A typical chromizing process involves packing the article to bechromized in a closed, sealed retort containing a source of chromiumatoms, such as pellets of ferrochrome alloy, and also containing ahalide such as granular ammonium chloride. The retort is heated in afurnace at a temperature of 1,800l,850 F. for about 6 to 8 hours; and agaseous chrome halide forms and reacts with the surface of the steelarticle causing iron atoms on the surface to be replaced by chromiumatoms. After obtaining the desired chromized surface, the article iscooled to room temperature, either inside or outside of the furnace.

A chromized steel product should have a carbon content as low aspossible (e.g., 0.3 carbon or less) to minimize formation, on thechromized surface, of chromium carbides which have an adverse effectupon the corrosion-resistance properties of a chromized steel product.However, the low carbon content results in relatively poor strength andtoughness (impact-resistance).

Chromized steel products should also contain carbon stabilizers, such astitanium, to tie up whatever carbon is present and prevent this carbonfrom combining with chromium on the chromized surface of the article.The low carbon content and the carbon stabilizers (which also areferrite stabilizers) favor the transformation of the microstructure ofthe product from an austenite phase to a ferrite phase at a relativelyhigh temperature during cooling from l,800-1,850 F. in the chromizingprocess; and the rate of nucleation of ferrite grains is low at the hightemperature. These factors cause a relatively large grain size in thefinished chromized product and restrict the amount of carbon stabilizer.The end result is a chromized steel product having a microstructureconsisting essentially of large grains of ferrite; and this type ofmicrostructure has poor strength and poor toughness.

SUMMARY OF THE INVENTION Embodiments of a process and product inaccordance with the present invention provide one or more of thefollowing improvements in a chromized steel product: improved strength;improved toughness; and smaller grain size.

A preferred embodiment of the invention utilizes the addition of 2.0-6.0percent nickel to the steel base before it is subjected to thechromizing process. This nickel addition increases the strength andtoughness of the chromized product; and, when the chromized product isfurnace-cooled following chromizing, the nickel addition causes asmaller grain size.

The nickel addition also permits an increased titanium content, which inturn also improves the strength of the product. Because titanium is aferrite stabilizer, in the absence of nickel it would be undesirable toincrease the titanium content of a steel undergoing chromizing beyondthat necessary to tie up the carbon and nitrogen in the product (e.g.,between four and five times the carbon content). This is because themore the ferrite is stabilized, the less likely the chances of improvingthe strength of the steel during cooling following chromizing. However,with the above-described nickel addition, the titanium content can beincreased without stabilizing the ferrite, because nickel is anaustenite former and offsets the ferrite-stabilizing effect of thetitanium.

Upon cooling, with a nickel content in accordance with the presentinvention, the chromized steel product undergoes a transfonnation fromaustenite to a micros tructure having, as a component, atransformation-induced dislocation substructure.

As used herein, the term substructure refers to structure which, thoughnot necessarily visually perceptible through an optical microscope, isperceptible with an electron microscope; and the term dislocation refersto the separation between a slipped crystalline region and an unslippedcrystalline region.

A dislocation may be induced by cold working or by transformation uponcooling, and the transfonnationinduced dislocation is the typeapplicable here. The extent of such a substructure, for a givencomposition depends upon the cooling rate: the faster the cooling rate,the more extensive the transformation-induced dislocation substructure.A normal, annealed structure generally has about 10 dislocations per cmand an embodiment in accordance with the present invention has a greaternumber of dislocations (essentially all the increase beingtransformation-induced) by at least one order of magnitude (10 per cm?)and preferably between 10 and 10 dislocations per cm.*.

In accordance with the present invention, waterquenching the steel,after chromizing at 1,8001,850 F. for 6-8 hours, would produce the mostextensive transformation-induced dislocation substructure, while coolingin the furnace would produce the least, although even cooling in thefurnace at the lower end of the specified nickel range (2.0 wt. wouldresult in a perceptible degree of increased dislocation substructure(e.g., an amount of dislocations about one order of magnitude greaterthan that existing when the steel is in an annealed condition).Substantially none of the carbon in the steel is associated with saidsubstructure, the carbon being tied up by the titanium.

Although nickel is a preferred addition agent for the purpose ofimproving the properties of the chromized steel product, other additionagents may be used, the criterion used in selecting the addition agentsbeing that the agent promotes transformation from austenite to amicrostructure containing an extensive transformation-induceddislocation substructure upon cooling following chromizing. Manganese isanother addition agent which could be used, although manganese does notimprove the toughness as does nickel. An extensivetransformation-induced dislocation substructure has a perceptibleincrease in dislocations compared to the number normally present in anannealed structure, e.g., an increase by one order of magnitude.

Other features and advantages are inherent in the structure claimed anddisclosed or will become apparent to those skilled in the art from thefollowing detailed description in conjunction with the accompanyingdiagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph plotting yieldstrength against nickel content for a chromized steel product producedin accordance with an embodiment of the present invention;

FIG. 2 is a graph plotting yield strength against titanium content for achromized steel product produced in accordance with an embodiment of thepresent invention; and

FIG. 3 is a graph showing the impact resistance transfonnationtemperature for chromized steel products having nickel contents inaccordance with embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A chromized steel product, inaccordance with the present invention, has a base composed of low carbonsteel, with the carbon content and the nitrogen content as low aspossible to minimize the formation of chromium carbides and chromiumnitrides, both of which have an adverse effect on the corrosionresistance of the surface of the chromized steel. A typical carboncontent is 0.03 wt. and a lower carbon content, if feasible, isdesirable. A typical residual nitrogen content is 0.004-0.005 wt. and alower nitrogen content, if feasible, is desirable.

When manganese is not purposely added as a strengthening agent, themanganese content is generally in the range of about 0.25-0.50 wt. witha preferred manganese range of 0.35-0.40 wt.

Phosphorus and sulphur are present in residual amounts only. Silicon andaluminum may be present as deoxidizing elements, silicon in the range0.2-0.3 wt. and aluminum in the range 0.02-0.05 wt. A preferable contentfor the silicon is about 0.25 wt. and a preferable .content for thealuminum is 0.03-0.04 wt.

To stabilize the carbon, the steel includes a carbon tieing elementselected from the group consisting essentially of titanium, vanadium,hafnium, zirconium, tantalum, columbium, etc.; and the amount ofcarbontieing element provided for this purpose is in atomic excess ofthe carbon. For example, assuming titanium is added to stabilize thecarbon, if the carbon content is 0.02, then the titanium added tostabilize the carbon should be greater than four times the carboncontent. This is because the ratio of the atomic weight of titanium(47.9) to the atomic weight of carbon (12.0) is about four to one. Thereshould be sufficient titanium to also tie up whatever residual nitrogenremains in the steel. A titanium to carbon weight ratio of about five toone should be sufficient for the purpose of preventing formation ofchromium carbides and chromium nitrides. Additional titanium, to improvestrength, may also be added; and this will be dealt with subsequently.

The strength and toughness of the product are both improved by addingbetween 2.0 and 6.0 wt. %'nickel, as a permissible range, with apreferable range of nickel being between 2.0 and 3.0 wt.

The following compilation of data compares the properties of a chromizedsteel product, having a base with a composition as described above, andthe properties of a chromized steel product having essentially the samebase composition but without nickel.

The steel base without nickel had the following composition:

Carbon 0.04 Manganese 0.32 Silicon 0.04 Aluminum 0.0 l 6 Titanium 0.36

A steel article having the above composition, subjected to a chromizingoperation at a temperature of l,800 F. for 8 hours, followed by eitherfurnace-cooling or air-cooling, develops the following properties:

FIG. 1 shows the improvement in yield strength for a steel producthaving additions of nickel in accordance with the present invention. Thesteels depicted in the graph of FIG. 1 contained 0.03 wt. carbon, 0.3wt. titanium, and 0.4-0.5'wt. manganese.

FIG. 3 shows the improvement in toughness or impact resistance, for afumace-cooled chromized steel having nickel added to the base inaccordance with the present invention. The carbon, titanium andmanganese contents for the steels of FIG. 3 are essentially the same asfor the steels depicted in FIG. 1.

With increased nickel content, the titanium content may also beincreased, without risking the retention of ferrite at the elevatedchromizing temperatures, without risking the formation of ferrite atrelatively high temperatures upon cooling, and without jeopardizing thetransformation of austenite to a microstructure containing an extensivedislocation substructure upon cooling. The increased titanium contentfurther increases the strength of the steel, as illustrated in FIG. 2.The carbon and manganese contents of the steels of FIG. 2 are the sameas those for the steels of FIGS. 1 and 3. The nickel content of thesteel of FIG. 2 is 2.9 wt. and the yield strength in all cases isgreater than 50,000 psi.

FIGS. 1-3 illustrate the improvement in strength and toughness forsteels which have been heated at l,800

F. and then fumace-cooled or air-cooled. For a chromized steel having2.0-6.0 wt. nickel in the base, the yield strength is at least about30,000 psi; and the ductile-brittle transition temperature is no greaterthan about 25 F. for the furnace-cooled product. If the chromized steelis water-quenched after chromizing at 1,800 E, some of the propertiesare further improved as shown in the following table.

Table II composition 0.2% offset tensile yield strength strengthelongation reduction Ni Ti PSI PSI in 2 inches in area Table III GrainSize, in Microns Wt. Nickel (and ASTM Scale) When the steel isair-cooled from l,800 F., the grain size is independent of the nickelcontent. Thus adding 2.0-6.0 wt. nickel and furnace-cooling decreasesthe grain size to no greater than about 17 microns.

After the steel has been chromized at a temperature of l,800-1,850 F.for 6-8 hours, the steel should be cooled under conditions-which preventoxidation of chromium. For example, the cooling may be conducted in aninert or reducing atmosphere, if the chromized product is to bewater-quenched or is to be removed from the furnace and retort at thechromizing temperature of l,800l,850 F. An alternative is to fumacecoolthe chromized steel product within the retort until it has cooled to atemperature at least as low as 1,000 F. (the temperature above whichchromium is oxidized) and then remove the chromized steel product fromthe retort and the furnace and cool it at a rate at least as fast aair-cooling. The chromized steel product may be permitted to cool in thefurnace until the temperature of the product is 200-300 F.

No matter the cooling rate to which the chromized steel product has beensubjected, when the base has a composition in accordance with thepresent invention the resulting microstructure will contain at leastsome transformation-induced dislocation substructure. If the product iswater-quenched, and if the cross section is small enough, themicrostructure will contain more of said dislocation substructure. Forslower cooling rates, or for relatively large cross sections, or both,the amount of said dislocation substructure will be less.

The strength of the chromized steel product may also be increased byadding manganese to the base in sufficient amount to raise the manganesecontent of the steel base to between 0.6 and 6.0 wt. The composition ofthe base is otherwise essentially the same as that utilizing nickel asthe element for improving strength and toughness, except that no nickelis present and manganese is present as described in the precedingsentence.

While manganese, like nickel, increases the strength of a chromizedsteel product, manganese, unlike nickel, does not increase thetoughness. A chromized steel product, in accordance with the presentinvention, and having a manganese content between 0.6 and 6.0 wt. has ayield strength exceeding 30,000 psi. The microstructure of a chromizedsteel product, containing manganese in accordance with the presentinvention, has a transformation-induced dislocation substructure, theextent of this substructure depending upon the cooling rate to which thechromized steel product has been subjected after the chromizing process.

Thus, in situations where improved strength is required but toughness isnot a significant factor, an embodiment of the present inventionutilizing manganese in amounts between 0.6 and 6.0 wt. is satisfactory.

Increased manganese content decreases the grain size; and increasing thecooling rate, with manganese present, also decreases the grain size. Inthis connection, reference is made to the following table.

TABLE IV Grain Size, Cooling Rate in Microns Manganese From l,800 F.(and ASTM Scale) 1.48 Furnace Cool 47 (5.5) L48 Air Cool 39 (6.0) 3.08Furnace Cool 37 (6.2) 3.08 Air Cool 31 (6.7)

Thus, air-cooling enhances the effect of manganese in reducing the grainsize, as compared to furnace-cooling; whereas, in contrast, air-coolinghas no effect on the grain size when nickel is used to improve thestrength and toughness of the chromized steel article.

However, although both manganese and nickel decrease the grain size,only nickel improves the toughness of the chromized steel product.

In accordance with other embodiments of the invention, other elementswhich favor the formation of a transformation-induced dislocationsubstructure upon cooling from 1,800-l ,850 F., may also be used toimprove the strength of the chromized steel product. For example, about3.0 wt. or more molybdenum may be used.

The invention is applicable to virtually all types of steel products:sheets, strips, bars, shapes, forgings, castings, extrudings, etc.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. A chromized steel product having a strong, tough steel base and adiffusion alloyed chromium surface, said base consisting essentially of:

no greater than 0.03 wt. carbon;

0.25-0.50 wt. manganese;

a carbon stabilizer, in atomic excess of the carbon, selected from thegroup consisting of titanium, vanadium, columbium, tantalum, halfniumand zirconium;

2.0-6.0 wt. nickel;

and the balance consisting essentially of iron;

said base having a microstructure comprising an extensivetransformation-induced dislocation substructure essentially withoutcarbon;

said carbon stabilizer being present in a percentage sufficiently smallto permit the formation of said microstructure.

2. A chromized steel product as recited in claim 1 and having a yieldstrength exceeding 30,000 psi.

3. A chromized steel product as recited in claim 2 wherein said producthas a Charpy V-notch ductilebrittle transition temperature no higherthan about 25 F.

4. A chromized steel product as recited in claim 1 wherein:

the average grain size of the base is no greater than about 17 microns.

5. A chromized steel product as recited in claim 1 and having a siliconcontent no greater than 0.3 wt.

6. A chromized steel product having a strong, tough steel base and adiffusion alloyed chromium surface, said base consisting essentially of:

no greater than 0.03 wt. carbon; 0.25-0.50 wt. manganese; titanium. inatomic excess of said carbon, up to L0 wt. 2.0-6.0 wt. nickel; and thebalance consisting essentially of iron; said base having amicrostructure comprising an extensive transformation-induceddislocation substructure essentially without carbon. 7. A chromizedsteel product as recited in claim 6 wherein:

said titanium content is substantially greater than five times thecarbon content. 8. A chromized steel product as recited in claim 7wherein:

said titanium content is at least 0.2 wt. said steel product having ayield strength greater than 50,000 psi.

9. A chromized steel product as recited in claim 6 wherein said producthas:

a Charpy V-notch ductile-brittle transition temperature no higher thanabout -25F; and a yield strength exceeding 30,000 psi. 10. A chromizedsteel product as recited in claim 6 wherein:

the average grain size of the base is no greater than about 17 microns.

1. A chromized steel product having a strong, tough steel base and adiffusion alloyed chromium surface, said base consisting essentially of:no greater than 0.03 wt. % carbon; 0.25-0.50 wt. % manganese; a carbonstabilizer, in atomic excess of the carbon, selected from the groupconsisting of titanium, vanadium, columbium, tantalum, halfnium andzirconium; 2.0-6.0 wt. % nickel; and the balance consisting essentiallyof iron; said base having a microstructure comprising an extensivetransformation-induced dislocation substructure essentially withoutcarbon; said carbon stabilizer being present in a percentagesufficiently small to permit the formation of said microstructure.
 2. Achromized steel product as recited in claim 1 and having a yieldstrength exceeding 30,000 psi.
 3. A chromized steel product as recitedin claim 2 wherein said product has a Charpy V-notch ductile-brittletransition temperature no higher than about -25* F.
 4. A chromized steelproduct as recited in claim 1 wherein: the average grain size of thebase is no greater than about 17 microns.
 5. A chromized steel productas recited in claim 1 and having a silicon content no greater than 0.3wt. %.
 6. A chromized steel product having a strong, tough steel baseand a diffusion alloyed chromium surface, said base consistingessentially of: no greater than 0.03 wt. % carbon; 0.25-0.50 wt. %manganese; titanium, in atomic excess of said carbon, up to 1.0 wt. %;2.0-6.0 wt. % nickel; and the balance consisting essentially of iron;said base having a microstructure comprising an extensivetransformation-induced dislocation substructure essentially withoutcarbon.
 7. A chromized steel product as recited in claim 6 wherein: saidtitanium content is substantially greater than five times the carboncontent.
 8. A chromized steel product as recited in claim 7 wherein:said titanium content is at least 0.2 wt. %; said steel product having ayield strength greater than 50,000 psi.
 9. A chromized steel product asrecited in claim 6 wherein said product has: a Charpy V-notchductile-brittle transition temperature no higher than about -25*F; and ayield strength exceeding 30,000 psi.